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Secondhand smoke, once considered a mere nuisance, has proved to be far more harmful. It can lead to lung cancer and heart disease, exacerbate asthma and cause pneumonia and bronchitis in babies. Now, a new study links it to another serious condition: dementia in adults.

A study published Feb. 13 in the British Medical Journal found a significant increase in the risk of dementia and other forms of cognitive impairment in people over 50 who have been exposed to high levels of secondhand smoke. (See pictures of vintage pro-smoking advertisements.)

Previous research has linked active smoking with cognitive impairment. But this is the first large-scale study to associate secondhand-smoke exposure to dementia and other neurological problems in older populations.

In the study, researchers at the University of Cambridge, Britain's Peninsula Medical School and the University of Michigan tested saliva samples from nearly 5,000 non-smoking adults over the age of 50 for cotinine — a by-product of nicotine — high levels of which would signal exposure to secondhand smoke. Participants in the study also provided a detailed smoking history. The researchers then used established neuropsychological tests to assess brain function and cognitive impairment.

They found that patients with high levels of cotinine were 44% more likely to show signs of cognitive impairment than those with very low levels. There was also "an exposure-response gradient" between cotinine concentration and poor mental performance: the more cotinine in a subject's saliva, the worse that subject performed on tests measuring mental agility, memory and clear thinking. (Read "The Year in Medicine 2008: From A to Z.")

Alzheimer's and dementia experts think that the mediating factors between secondhand smoke and cognitive impairment could be heart disease and stroke; secondhand-smoke exposure raises the risk of heart disease and stroke, which in turn raise the risk of dementia.

"One potential mechanism could be that smoke disrupts the way in which our blood vessels carry blood to the brain," says Sarah Day, head of public health for Britain's Alzheimer's Society. "A type of dementia called vascular dementia is caused by minute hemorrhages in the brain. If smoke is having an effect on the cells in the blood vessel walls, that's a pretty good explanation as to why secondhand smoke would have an effect." (Read "Mild Exercise May Counter Dementia.")

Responding to the research, Alzheimer's campaigners joined the study's authors in calling for public-smoking bans in cities, states and countries that have so far resisted them.

A wave of recent research has linked secondhand smoke to health problems; many of these studies were made possible by the implementation of smoking bans throughout the developed world. A recent study, for example, showed that the risk of heart problems dropped in both smokers and nonsmokers following Scotland's smoking ban in 2006. Many U.S. states are still without smoking bans, however, as is almost all of the developing world. But opponents of such legislation are now fighting a losing battle, according to Dr. Iain Lang of Peninsula Medical School, a co-author of the study.

"I [think] this is a final nail in the coffin for those who try to suggest there is no harm involved in secondhand smoke," he says.

In an editorial accompanying the study, Mark Eisner, an associate professor of medicine at the University of California, San Francisco, commended the study but pointed out that cotinine only remains in saliva for two to three days after exposure to smoke, which is problematic as cognitive impairment in the elderly develops over many years. He called for further study into the effects of cumulative lifetime exposure to secondhand smoke to confirm the link.

But in a strongly worded conclusion, Eisner pointed out that some of the most important breakthroughs for human health have come from altering environmental factors and that even in an era of "groundbreaking science aimed at decoding the human genome...millions of people worldwide continue to die from passive smoking." Calling for further antismoking legislation, he argued that in the fight against illness and premature death, "we should not forget the 'low-tech' intervention of clearing the air we breathe."

See pictures of the world's most polluted places.

See the top 10 medical breakthroughs of 2008.

• Eben Harrell